1. Field of the Invention
The invention relates to a hydraulic pressure control apparatus for a vehicular hydraulic power transmission device which includes a lock-up clutch whose operating state can be changed using a hydraulic pressure difference between an engagement side oil chamber and a disengagement side oil chamber.
2. Description of the Related Art
A vehicle is known, which includes a lock-up clutch that can mechanically connect an input side and an output side of a hydraulic power transmission device such as a torque converter and a fluid coupling such that torque can be directly transmitted. The lock-up clutch is disposed between a front cover that is an input side rotational member of the hydraulic power transmission device, and a turbine or a pump that is an output side rotational member thereof. The lock-up clutch divides a space between the front cover and the turbine or the pump into a disengagement side oil chamber on the front cover side and an engagement side oil chamber on the turbine or the pump side. Thus, a contact state between the lock-up clutch and the front cover, that is, an operating state of the lock-up clutch is controlled so as to be changed according to a hydraulic pressure difference between the engagement side oil chamber and the disengagement side oil chamber, which is obtained by subtracting the hydraulic pressure in the disengagement side oil chamber from the hydraulic pressure in the engagement side oil chamber (i.e., the hydraulic pressure in the engagement side oil chamber—the hydraulic pressure in the disengagement side oil chamber). When hydraulic oil in the disengagement side oil chamber is drained through a first oil passage which communicates with the disengagement side oil chamber, and hydraulic oil is supplied to the engagement side oil chamber through a second oil passage which communicates with the engagement side oil chamber, the hydraulic pressure in the engagement side oil chamber becomes higher than that in the disengagement side oil chamber, that is, the hydraulic pressure difference becomes positive. At this time, the lock-up clutch is engaged with the front cover through frictional material. In other words, the lock-up clutch is engaged with increasingly larger engagement force, and the lock-up clutch is completely engaged. When the hydraulic oil is supplied also to the disengagement side oil chamber, and accordingly the hydraulic pressure difference is decreased, the lock-up clutch is semi-engaged, that is, the lock-up clutch is placed in a slip state.
When the lock-up clutch is frequently placed in the slip state, frictional heat that is generated due to the slip increases. The durability of the frictional material may deteriorate due to heat deterioration caused by the frictional heat. Accordingly, a technology is proposed, in which a third oil passage that communicates with the engagement side oil chamber is provided, the hydraulic oil is circulated from the second oil passage to the third oil passage, and the lock-up clutch is cooled due to the circulation so as to improve the durability of the frictional material. One example of the technology is disclosed by Japanese Patent Laid-Open Publication No. 5-187541. In the technology, a valve through which the hydraulic oil flows from the third oil passage that communicates with the engagement side oil chamber to a drain oil passage is provided, and opening/closing of the valve is controlled. For example, a sensitive opening/closing valve called a bimetal orifice is provided. In the case where the temperature of the hydraulic oil increases, and becomes higher than a set temperature when the lock-up clutch is engaged with the front cover, the bimetal orifice is opened and part of the hydraulic oil in the engagement side oil chamber is drained. The hydraulic oil is circulated from the second oil passage so as to return to the second oil passage via the engagement side oil chamber, the third oil passage, and the drain passage. Thus, the lock-up clutch is cooled due to the circulation of the hydraulic oil so as to improve the durability of the frictional material. In addition to the aforementioned patent document, technologies concerning the lock-up clutch are disclosed in Japanese Patent Laid-Open Publication No. 8-14381, Japanese Patent Laid-Open Publication No. 8-4891, Japanese Patent Laid-Open Publication No. 7-180768, Japanese Patent Laid-Open Publication No. 4-4354, Japanese Patent Laid-Open Publication No. 10-89462, and the like.
However, in the case of the aforementioned bimetal orifice, only opening/closing thereof can be controlled, and a flow rate of the hydraulic oil when the orifice is opened cannot be controlled. The aforementioned hydraulic pressure difference may be changed depending on the flow rate. Thus, the control operation for the lock-up clutch, for example, the slip control operation may not be appropriately performed.